German Patent No. DE 40 26 228 C1 describes a test device for determining the hydraulic leakage rate of a fuel injection valve, in particular for internal combustion engines. This test device is situated obliquely in a vessel filled with liquid, into which there also opens a compensating vessel that can be separated by a blocking valve, as well as riser pipe fashioned as a glass capillary, the level in the capillary being illuminated by a light source and the determination of the level being acquired by a linear line scan camera. After the measurement of the initial level in the capillary, the injection valve is charged with a test pressure, and after the measurement time the change in the level in the capillary is measured using the line scan camera. The leakage rate of the injection valve can be calculated from the measurement time, the cross-sectional surface, and the displacement path in the capillary. Such a test device has a complicated design and does not provide absolutely precise measurement results.
Moreover, in order to determine the leakage rate of an injection valve it is known to use a test device in which the injection valve is placed into a test body, the injection valve being charged with a test liquid under pressure and the quantity leaked into the test body being borne away by ambient air. The leakage rate can then be determined by measuring and evaluating the concentration of the test liquid in the ambient air stream using an analyzer. Such a test device has the flaw that the leakage quantity, in particular in the case of liquids having low vapor pressures, is converted to the vapor phase incompletely and in an undefined manner, thus falsifying the determined measurement value or its curve over time. Disadvantageously, in this way during one and the same measurement process leakage flows are indicated that are in part too low and in part too high. Moreover, a partially or completely produced vapor phase can condense after the leakage point, in particular at cooler locations on the test body or at other locations of the test device, which can also falsify the measurement value. Here as well, there is the danger of measurement of a too-low and/or too-high leakage flow, or of a leakage flow that fluctuates strongly throughout. This disadvantage occurs in particular in the case of measurements using liquids or liquid mixtures that condense with a particular pressure, temperature, or concentration under measurement chamber conditions.